The present invention relates generally to the art of voltage level detectors, and more specifically is concerned with the detection of voltage level crossings of the modulating signal component of a frequency modulated signal.
In a frequency modulated signal, such as an FM broadcast signal, the information is represented in the frequency domain. The original information waveform, e.g. an audio signal waveform, is referred to as the modulating signal and in that form the information is represented by the varying voltage characteristics of the waveform. When the original information waveform is used to frequency-modulate a carrier signal, however, the information is translated so that it is represented by the variance in frequency of the carrier signal about the carrier center frequency. Thus, for instance, a modulating signal which varies between +50 volts and -50 volts in accordance with the information it represents will, when used to frequency-modulate a carrier signal having a frequency of 300 MHz, produce an FM signal which swings correspondingly between a frequency of 350 and 250 megacycles about the carrier center frequency of 300 MHz.
It is desirable in some applications using FM signals to know when the modulating component of the FM signal either crosses a zero voltage level or some other predetermined voltage level. This cannot be done by directly measuring the voltage level of the FM signal, and so the FM signal must first be converted into a form where it is possible. Typically, the FM signal is first demodulated and the resulting signal applied to a discriminator, which produces a voltage-varying signal, which signal in turn is applied to a level detector. This process is expensive and complex, slow and often imprecise.
In view of the above, it is a general object of the present invention to provide a voltage level detection circuit which overcomes the disadvantages of the prior art stated above.
It is another object of the present invention to provide such a circuit which provides an indication of voltage level crossings by the modulating component of a frequency modulated signal without the demodulation thereof.
It is a further object of the present invention to provide such a circuit wherein the value of the voltage level may be varied.